Satellite positioning systems are now well-known in the art. Such systems, for example, NAVSTAR-GPS are rapidly being used for determination of the geocentric position of mobile units, such as water and land vehicles, aircraft and survey equipment to name a few.
In aircraft, GPS systems are being utilized for navigation, flight control, and air space control. These GPS systems may operate independently or in combination with inertial reference systems or attitude heading reference systems in order to provide information during an aircraft flight mission. Global positioning systems similar to NAVSTAR commonly use a GPS receiver, located on a mobile unit, for receiving satellite information signals transmitted from a plurality of satellites. Each GPS satellite transmits an information signal containing data that allows a user to determine the range or distance between selected GPS satellites and the antenna associated with the mobile unit's GPS receiver. These distances are then used to compute the position of the receiver unit using known triangulation techniques. For example, in the NAVSTAR-GPS system, a mobile unit with a GPS receiver, such as an aircraft, detects a pseudo random code contained in a given GPS satellite information signal and derives therefrom the "elapsed time" or time delay between the transmission of the signal and its reception at the GPS receiver. From this time delay, the GPS receiver can derive the range between the GPS receiver antenna and the satellite, sometimes referred to as the pseudo-range or pseudo-range measurements. Herein, the GPS receiver's position, or the mobile unit's position, generally refers to the corresponding antenna position.
In addition, as part of the NAVSTAR-GPS system, each satellite information signal also contains precise ephemeris data and course almanac data which both describe the corresponding satellite orbital trajectory in earth centered space as is well known in the art. The coordinates of the satellite's orbital position may be derived from either the ephemeris data or the almanac data. The geocentric position of the satellite may be calculated with a higher degree of precision from the ephemeris data than is possible with the almanac data. However, because the ephemeris data precisely describes the satellite trajectory at the moment of transmission of the satellite information signal, it is only valid for a few hours thereafter, as is well known.
It should be understood that the mobile unit's three-dimensional geocentric position in World Geodetic System Coordinates may be determined using either the ephemeris data or almanac data received from four or more satellites. Herein, it should be recognized by those skilled in the art that the World Geodetic System is an earth-centered, earth-fixed geocentric coordinate system, which may be converted to any other coordinate system as required by the user. Sometimes the aforementioned coordinate system is referred to as the WGS84 earth-centered, earth-fixed, rectangular coordinate frame. Herein, the World Geodetic System Coordinates should be presumed, and position refers to this three dimensional WGS84 coordinate system.
In order to determine the position of the GPS receiver unit, a minimum of four satellite signals are required rather than the expected three. This is so, since the GPS receiver includes a receiver clock which is not as accurate as the atomic clock of the satellites. Therefore, receiving satellite information signals from four different satellites provides a complete solution which permits the correction of any receiver clock error as is well understood in the art. Herein, the corrected receiver clock time is referred to as the receiver time. Thus, if signals from four or more satellites are available to the GPS receiver unit, the geocentric position of the receiver may be determined within approximately one-hundred meters of its "true" geocentric position. Herein, the receiver position derived by the triangulation technique using data from multiple satellites is referred to as the "estimated position". The accuracy of the estimated position of the receiver unit is dependent upon many factors including, among others, atmospheric conditions, selective availability, and the relative position of the satellites with respect to the line of sight view of the satellites.
Associated with a GPS estimated position is a "position error bound" as particularly defined by accepted GPS system standards which have been developed by the Radio Technical Commission for Aeronautics (RTCA), in association with aeronautical organizations of the United States from both government and industry. The RTCA has defined the phrase "GPS system integrity" as the ability of a GPS system to provide timely warnings to users when the GPS system should not be used for navigation. "System integrity" is particularly identified in a document entitled "Minimum Operational Performance Standards for Airborne Supplemental Navigation Equipment Using Global Positioning System (GPS)", document number RTCA/DO-208, July 1991, prepared by: SC-159, beginning at section 1.5. As described therein, GPS is complicated in that it is a four-dimensional system involving three components of position and one time component. As also described in the aforesaid RTCA publication, the signal-in-space error transforms into a horizontal position error via a relatively complex function of the satellite geometry at any given moment. The GPS integrity system must interpret the information it has about the pseudo-range errors in terms of the induced horizontal and vertical position errors, commonly referred to as the "position error bounds", and then make a decision as to whether the position error bounds are outside the allowable radial error, specified for a particular phase of the flight mission in progress. The allowable error is referred to as the "alarm limit", herein referred to as the integrity alarm limit. If the horizontal position error bound is found to exceed the integrity alarm limit, a timely warning must be issued by the GPS system receiver unit or sub-system.
Two rather distinct methods of assuring GPS integrity have evolved as civil use of GPS has progressed. One is the Receiver Autonomous Integrity Monitoring (RAIM concept, and the other is the ground monitoring approach that goes under the name "Wide Area Augmentation System" (WAAS). The intent of both of these methods is the calculation of the position error bound with regard to the current GPS estimated position so that it may be compared with the alarm limit associated with a particular phase of the flight mission.
The Receiver Autonomous Integrity Monitoring System employs a self consistency check among the measurements, more specifically, the pseudo-range measurements. Satellite redundancy is required to perform a self-consistency check on an instantaneous basis. Thus, five satellites must be in view, i.e., satellite signals received and pseudo range measurements calculated by the GPS receiver. If fewer than five satellites are in view the value of the predicted position error bound will be infinite. Also, there are constraints on the satellite constellation geometry that must be met if the check is to be effective in the presence of noise. Generally speaking, a satellite constellation with many satellites in view, permits a robust integrity monitoring system. Conversely, a satellite constellation having only a few satellites in view may limit the availability of an integrity monitoring system. There may be short periods when a good consistency check is not possible (less than 5 satellites in view). The main feature of RAIM is that it is completely self-contained and relatively easy to implement in software.
Examples of RAIM may be found in the aforementioned RTCA publication, Appendix F, and another is described in an article entitled "Implementation of a RAIM Monitor in a GPS Receiver and an Integrated GPSMU" by Mats Brenner located at page 397 in the Proceedings of ION GPS-90, Third International Technical Meeting of the Satellite Division of the Institute of Navigation, Sept. 19-21, 1990.
GPS systems which incorporate RAIM output a position error bound which represent the probabilistic radial errors of the navigation solution, namely the GPS estimate position of the receiver unit. Currently, RAIM may generate several numbers including, a horizontal position error bound, a vertical position error bound, and a spherical position error bound for the current time, i.e., the instant of time the GPS measurements occurred. A calculation for the horizontal position error bound is further described in equation 21 of the aforementioned Mats Brenner paper. Herein the term, position error bound, will be used to denote either separately or the combination of the horizontal. vertical and spherical position error bounds.
Once calculated, the position error bound is used to determine if the pilot can rely on the derived GPS estimated position for the current phase of flight. It should be recognized that some interpolation may be required dependent upon the receiver's ability to simultaneously receive a plurality of satellite information signals as is well understood in the art.
The allowable integrity alarm limit value, may change depending on the phase of the aircraft flight mission. For instance, if a pilot is flying in the termninal phase, the integrity alarm limit may be less stringent than if the pilot is in the approach phase of flight. If the pilot is to transition from the terminal phase of flight to the approach phase of flight, the pilot needs to know whether the current position error bound is sufficient to allow the pilot to rely upon the GPS solution to make the transition.
GPS systems may provide the pilot a predicted position error bound based on the estimated time of arrival (ETA) at the destination. In this scenario however, the predicted position error bound may be unreliable, or overly optimistic for several reasons. First, the aircraft may arrive at a time other then original ETA. Thus, satellites predicted to be available at the original ETA may not be available at the revised ETA. Secondly, a satellite information signal that was predicted to be available at the original ETA, might not be able to be obtained by the GPS receiver when predicted. This could be due to atmospheric effects, satellite failure, signal blockage, or receiver shielding. If the position error bound is not within the specified integrity alarm limit once the pilot has committed to the approach phase of the flight, a pilot may have to execute a missed approach.